The introduction of foreign genes into a retroviral shuttle vector offers the opportunity of infecting a variety of human cell types at high frequency. The coding region of the v-Ha-ras oncogene was cloned in both orientations into the murine retroviral shuttle vector pZip-neo SV(X). Plasmid DNA of bacterial clones containing the Ha-ras coding region was isolated and transfected into NIH 3T3 cells by the calcium phosphate precipitation method. One out of six clones transfected was biologically active in induction of transformed foci. The plasmid DNAs (pZip-neo-ras and pZip-neo-sar orientation) were transfected into Psi-2 cells by the calcium phosphate precipitation method. G418 resistant clones of transfected Psi-2 cells were isolated and their supernatant was titered for virus production. Virus titers of about 10 to the fifth power virus/ml were achieved. Subsequent infection of Psi-am cells with recombinant virus produced from Psi-2 cells yielded virus stocks of 5 x 10 to the third power - 10 to the fourth power virus/ml.